Caffeine adsorption

ABSTRACT

In the decaffeination of green coffee beans with an aqueous medium, caffeine is removed from the aqueous medium which also contain non-caffeine green coffee solids, contact with substantially neutral active carbon. Thereafter the medium may be recycled to extract further amounts of caffeine from the same or another batch of green coffee beans. 
     Other features of the invention are described in the specification.

This invention is concerned with the recovery of caffeine from aqueousmedia.

In the preparation of decaffeinated coffee, two basic approaches areavailable for the extraction of caffeine from the beans. According tothe first, green (unroasted) coffee beans are extracted with water or anaqueous solution containing non-caffeine green coffee solids, theaqueous extract is separated from beans, caffeine is removed from theextract, usually by solvent extraction or adsorption on a solidadsorbent and the caffeine-free extract may be returned to the coffeebeans. The second approach involves direct contact between the beans andthe caffeine solvent, usually methylene chloride, and evaporation of thesolvent leaving behind the caffeine. Of the various solid adsorbentsthat have been proposed, activated carbon offers certain advantages oversubstances such as polymeric resins because of its ready availabilityand ease of regeneration. However, it has been observed that contactbetween the aqueous extract and active carbon frequently leads to anincrease of pH which is associated with a deterioration in the colourand flavour of the coffee beans.

It has now been found that the undesirable increase of pH may beconsiderably diminished if the activated carbon used shows asubstantially neutral reaction on dispersion in water.

The invention thus provides a process for the recovery of caffeine froman aqueous solution containing caffeine extracted from green coffeebeans and non-caffeine green coffee solids, which comprises contactingthe solution with substantially neutral activated carbon and separatingthe activated carbon, with caffeine adsorbed thereon, from aqueoussolution of reduced caffeine content. The term "substantially neutral"used to describe the activated carbon means that when the carbon isimmersed in water the pH value is substantially unchanged. Carbon havingthis property may be obtained either by acid washing of thermallyactivated carbon followed by rinsing with water to neutrality, or byneutralisation of acid-activated carbon with an aqueous alkali followedby rinsing with water to neutrality.

The aqueous caffeine solution is obtained by conventional methods,involving contact of green coffee beans with water for a period timesufficient to reduce the caffeine content of the beans to the desiredlevel. The contacting may be effected in a counter-current system, usingan arrangement similar to those used for extracting roasted coffee inthe preparation of coffee extract, or batch contact may be employedwherein a fixed volume of water is continuously recycled to a fixedweight of coffee beans, caffeine being removed from the extract at eachcycle prior to its being returned to the beans. In the counter-currentsystem, the aqueous extraction medium containing caffeine andnon-caffeine green coffee solids encounters coffee of progressivelyhigher caffeine content. On leaving the system, the caffeine-ladenextract is contacted with the adsorbent, preferably again in acounter-current column system. The main factors affecting the operationare temperature, the ratio of extraction liquid to coffee, the ratio ofcarbon to coffee, time and liquid velocity, each of which may be adaptedto the degree of decaffeination desired. In carrying out the processaccording to the invention, the neutral activated carbon may be usedeither in the batch or the counter-current continuous system. In bothcases provision is usually made for continuity of operation byduplicating the beds of adsorbent so that one or more may be renewedwhilst the others are on stream.

Decaffeination of green coffee beans is preferably effected withdeionised water; the water to coffee ratio is by no means critical, butrather is determined having regard to practical considerations imposedby industrial operations. Excessive volumes are thus avoided, as alsowater to coffee ratios which do not provide for adequate caffeineextraction. It has been found that in general a weight ratio of water togreen coffee of at least about 3 parts of water per part of coffee givessatisfactory results.

In one preferred embodiment, green coffee beans are contacted as astatic bed, in a column, or in a suitable tumbler or like extractor. Inboth cases, the weights of coffee and water are constant, with thecaffeine-laden water withdrawn from the column or extractor beingdecaffeinated with neutral activated carbon prior to its being recycled.The weight of carbon is usually 20 to 30% of the weight of green coffeebeing decaffeinated. The total contact time will depend, inter alia, onthe water/coffee ratio, temperature and the degree of decaffeinationdesired. In most cases it is 3 to 10 hours. The temperature ispreferably in the range 60° to 90° C. Lower temperatures are usuallyavoided as the risk of microbial growth, especially with long contacttimes, is increased, resulting in fermentation of the sugars present inthe extract. In addition, the rate of caffeine diffusion from the beansdecreases with temperature. Above 90° C., with long contact times,flavour may be impaired and as a practical matter it is difficult tomaintain these temperatures without resort to pressurised equipment.

In another preferred embodiment of the invention, applied to continuousextraction and decaffeination of green coffee beans, a counter-currentsystem is used.

The green coffee is extracted using an arrangement similar to those usedfor extracting roasted coffee. The fresh water entering the mostexhausted extractor is normally at a temperature in the range 90° to120° C. Deionised water is used for preference. The water to coffeeratio is not critical but is generally between 3:1 and 15:1 by weight.The number of extractors and cycle time are chosen to give the desireddegree of decaffeination. Up to eight extractors, in series, may be usedwith a cycle time of between 15 and 120 min.

The aqueous extract containing caffeine and nonaffeine green coffeesolids may be concentrated before being decaffeinated by contacting withneutralised, activated carbon, for example in a counter-current system.In this system the carbon is contained in several columns and theextract passes through these columns in series. Periodically the mostsaturated column is removed from the system and one containing freshcarbon added. The temperature in the columns is preferably between 60°and 90° C. The number of columns, the cycle time for each column and theresidence time of the extract are chosen to achieve the degree ofdecaffeination desired and to minimise the quantity of carbon used. Theweight of carbon is usually 10-20% of the weight of green coffee beingdecaffeinated.

In the batch/recirculation system, contact tempeture between carbon andextract should desirably be at least about 60° C. to avoidmicrobiological problems, whereas in a column arrangement the inlettemperature should be a little higher for the same reason. Nowell-defined relationship between temperature and caffeine adsorptionhas been observed.

Since the activated carbon adsorbs acids as well as caffeine, the pH ofthe aqueous extract rises, reaching a maximum of 6.0 to 7.5. However, asthe operation proceeds, the pH falls again so that when, for example,97% decaffeination is attained, it has risen by less than 1 unit overthe natural pH of an aqueous green bean extract. In contrast, the pH ofa green bean extract contacted with non-neutralised activated carbonrises to above 9.0, which possibly explains the deterioration in colourand flavour.

By way of illustration, the results obtained when different activatedcarbons are employed in the batchwise manner described are given below.A deionised water/coffee ratio of 4:1 by weight is used, 24% carbonbased on the weight of green coffee and the temperature of the systemmaintained at 70° C. during 8 hours.

    ______________________________________                                                        pH of carbon                                                                  in distilled                                                                             Max. pH  Final pH                                                  water at   of extract                                                                             of extract                                Carbon          20° C.                                                                            at 20° C.                                                                       at 20° C.                          ______________________________________                                        A.  Thermally activated,                                                          non-neutralised                                                               A1              10.8       9.4    7.2                                         A2              11.2       9.6    7.0                                         A3              9.5        9.5    7.1                                     B.  Thermally activated,                                                          neutralised                                                                   B1              7.5        6.7    6.2                                         B2              7.7        6.2    5.9                                         B3              8.5        7.5    6.1                                         B4              7.4        6.5    5.5                                     C.  Acid activated                                                                neutralised                                                                   C1              6.2        6.5    6.4                                     ______________________________________                                    

When decaffeination is terminated, it is usually desirable, to avoidexcessive losses, to return the non-caffeine solids present in theaqueous extract to the decaffeinated green beans. Various techniques maybe used. For example, the beans may be pre-dried, usually to 10-45% byweight moisture and combined directly with the extract. Alternativelythe extract may be pre-concentrated, e.g. by evaporation, to a solidscontent of 15-55% by weight before contact with the beans. Satisfactoryreincorporation of the solids is obtained in 4 to 8 hours, preferably at60°-80° C. Desirably, the total amount of water present is such that thefinal moisture content of the beans does not exceed about 55% by weight.The "total" water is made up of the residual moisture of the pre-driedbeans and the water present in the aqueous extract. Afterreincorporation of solids the coffee is dried to a moisture content of5-12% by weight before being roasted. In a modification, the amount ofnon-caffeine solids contacted with the coffee beans may be less than theamount extracted during decaffeination.

Periodically the active carbon may be regenerated, usually by heating orsolvent extraction.

The invention is illustrated by the following Examples, in which allparts, ratios and percentages are expressed on a weight basis unlessotherwise stated.

EXAMPLE 1

Neutralised activated carbon is prepared by washing commercial thermallyactivated carbon with 2% hydrochloric acid followed by rinsing withdeionised water (pH=6.0) until the pH of the washings is constant at6.0. 2000 parts of green coffee beans are loaded into a tumblerextractor together with 7000 parts of deionised water. The temperatureis raised to (and maintained at) 80° C. Solution is withdrawn from theextractor and recirculated at a rate of 15000 parts/hour, passingthrough a column containing 230 parts of the neutralised activatedcarbon, before returning to the extractor. After one hour, a secondcolumn also containing 230 parts of neutralised activated carbon isconnected in series with the first.

After 5 hours' total operation, recirculation is stopped and theextract, having a pH of 6.2, is separated from the beans. The beans aredried to 20% moisture and mixed with the extract, which has first beenconcentrated to 20% solids. Mixing is continued for 6 hours at 65° C.Thereafter, the coffee, containing 45% moisture, is dried to a 8.5%moisture content.

The coffee is 97% decaffeinated and has a good appearance, similar tothe original, non-decaffeinated beans but somewhat darker in colour.When roasted and prepared as an infusion, the brew is described by apanel of trained tasters as being of good quality and colour. When milkis added to the cup, the brew assumes a red-brown colour, without tracesof greyness.

EXAMPLES 2 to 4

The coffee decaffeination procedure described in Example 1 is repeatedwith various modifications in materials and operating parameters. Theresults are summarised in Table I.

                  TABLE I                                                         ______________________________________                                                                                Ex-  %                                             Water/  Carbon       Total tract                                                                              De-                                   pH of*  coffee  as % of                                                                              Temp. time  final                                                                              caff. of                         Ex.  carbon  ratio   coffee °C.                                                                          hours pH   coffee                           ______________________________________                                        1    7.5     3.5     23     80    5     6.2  97                               2    7.5     3.5     23     65    7     6.2  97                               3    7.5     3.5     23     80    5     6.0  97                               4    7.4     4.0     25     65    8     5.5  97                               ______________________________________                                         *measured in distilled water at 20° C.                            

After decaffeination, the non-caffeine solids are combined with thedecaffeinated beans.

EXAMPLE 5

Green coffee beans are decaffeinated continuously by counter-currentextraction with an aqueous solution. 6 extractors in series are used,each containing 60 kg of green coffee. Extraction is carried out withdeionised water at 100° C. entering the most exhausted extractor. Anaqueous solution containing caffeine and non-caffeine green bean solidsis removed at 80° C. from the extractor containing the least exhaustedcoffee. The last extractor containing decaffeinated coffee is removedfrom the system and one containing fresh green coffee beans added onceper hour. The ratio of water to green coffee beans is 10:1 so that theflow rate of water entering the system is 600 lt/hr.

The extract coming from the least exhausted green coffee is passedcounter-currently through 3 columns in series, each column containing 35kg of neutralised, activated carbon prepared in a manner similar to thatdescribed in Example 1. The temperature in the columns is maintained at75° C. The most saturated carbon column is removed and a fresh one addedevery four hours.

The decaffeinated green beans are dried to 20% moisture and mixed withthe corresponding quantity of decaffeinated extract, which has firstbeen concentrated to 20% solids. Mixing is continued for 6 hours at 75°C. Thereafter, the coffee containing 45% moisture is dried to 9.0%moisture content.

Table II summarises the results of tasting by a trained panel ofbeverages obtained from coffee decaffeinated in accordance with Examples1, 2 and 5 and roasted. These coffees are compared to coffeesdecaffeinated in a manner similar to that described in Example 1, butusing the carbn employed in Example 1 which has not been acidneutralised and which gives a pH of 10.8 in distilled water (A) and acarbon which gives a pH of 9.5 in distilled water (B).

                  TABLE II                                                        ______________________________________                                                                                   He-                                     pH of*                   Colour +     donic                              Ex.  carbon  Aroma    Taste   milk    Rank scale                              ______________________________________                                        1    7.5     Good,    Good, full                                                                            Red     3    6.9                                             fresh    clean, ba-                                                                            brown                                                                 lanced                                                  2    7.5     Good,    Good, acid                                                                            Red     1    7.1                                             full     clean,  brown                                                                 flavoury                                                5    7.5     Fresh,   Natural,                                                                              Red     1    7.1                                             aromatic clean, mild,                                                                          brown                                                                 balanced                                                A    10.8    Weak     Sl. nutty,                                                                            Grey    4    4.2                                                      harsh, acid,                                                                          brown                                                                 papery                                                  B    9.5     Sl. nutty,                                                                             Aggressiye,                                                                           Greyish 5    4.1                                             Sl. bready                                                                             sl. nutty,                                                                            brown                                                                 old                                                     ______________________________________                                         *measured in distilled water at 20° C.                            

We claim:
 1. A process for the recovery of caffeine from an aqueoussolution containing caffeine extracted from green coffee beans andnon-caffeine green coffee solids which comprises contacting the solutionwith substantially neutral activated carbon and separating the activatedcarbon, with caffeine adsorbed thereon, from aqueous solution of reducedcaffeine content.
 2. A process according to claim 1 in which thecontacting is effected at a temperature of 60° C. to 90° C.
 3. A processaccording to claim 1 or claim 2 in which the caffeine content of theaqueous solution is reduced substantially to zero and the non-caffeinegreen coffee solids therein are combined with green coffee beans havinga reduced content of caffeine and of non-caffeine green coffee solids.4. A process for the removal of caffeine from green coffee beans whichcomprises contacting the beans with an aqueous medium selected from thegroup consisting of water, an aqueous solution of non-caffeine greencoffee solids and an aqueous solution of non-caffeine green coffeesolids containing a minor amount of caffeine, recovering aqueous mediumcontaining caffeine dissolved from the green coffee beans, removingcaffeine from the medium by contact with substantially neutral activatedcarbon, separating the activated carbon with caffeine adsorbed thereonfrom aqueous medium of reduced caffeine content and combiningnon-caffeine green coffee solids present in the aqueous medium ofreduced caffeine content with green coffee beans having a reducedcontent of caffeine and of non-caffeine green coffee solids.
 5. Aprocess according to claim 4 in which the caffeine content of theaqueous medium is reduced substantially to zero.